‘Hot-Fill’ applications impose significant and complex mechanical stress on a container structure due to thermal stress, hydraulic pressure upon filling and immediately after capping, and vacuum pressure as the fluid cools.
Thermal stress is applied to the walls of the container upon introduction of hot fluid. The hot fluid will cause the container walls to soften and then shrink unevenly, causing distortion of the container. The polyester must therefore be heat-treated to induce molecular changes resulting in a container that exhibits thermal stability.
Pressure and stress are acted upon the side walls of a heat resistant container during the filling process, and for a significant period of time thereafter. When the container is filled with hot liquid and sealed, there is an initial hydraulic pressure and an increased internal pressure is placed upon containers. As the liquid, and the air headspace under the cap, subsequently cool, thermal contraction results in partial evacuation of the container. The vacuum created by this cooling tends to mechanically deform the container walls.
Generally speaking, containers incorporating a plurality of longitudinal flat surfaces accommodate vacuum force more readily. Agrawal et al, U.S. Pat. No. 4,497,855 discloses a container with a plurality of recessed collapse panels, separated by land areas, which allows uniformly inward deformation under vacuum force. The vacuum effects are controlled without adversely affecting the appearance of the container. The panels are drawn inwardly to vent the internal vacuum and so prevent excess force being applied to the container structure, which would otherwise deform the inflexible post or land area structures. The amount of ‘flex’ available in each panel is limited, however, and as the limit is approached there is an increased amount of force that is transferred to the side walls.
To minimise the effect of force being transferred to the side walls, much prior art has focused on providing stiffened regions to the container, including the panels, to prevent the structure yielding to the vacuum force.
The provision of horizontal or vertical annular sections, or ‘ribs’, throughout a container has become common practice in container construction, and is not only restricted to hot-fill containers. Such annular sections will strengthen the part they are deployed upon. Cochran U.S. Pat. No. 4,372,455 discloses annular rib strengthening in a longitudinal direction, placed in the areas between the flat surfaces that are subjected to inwardly deforming hydrostatic forces under vacuum force. Akiho Ota et al U.S. Pat. No. 4,805,788 discloses longitudinally extending ribs alongside the panels to add stiffening to the container. Akiho Ota also discloses the strengthening effect of providing a larger step in the sides of the land areas. This provides greater dimension and strength to the rib areas between the panels. Akiho Ota et al, U.S. Pat. No. 5,178,290 discloses indentations to strengthen the panel areas themselves.
Akiho Ota et al, U.S. Pat. No. 5,238,129 discloses further annular rib strengthening, this time horizontally directed in strips above and below, and outside, the hot-fill panel section of the bottle.
In addition to the need for strengthening a container against both thermal and vacuum stress, there is a need to allow for an initial hydraulic pressure and increased internal pressure that is placed upon a container when hot liquid is introduced followed by capping. This causes stress to be placed on the container side wall. There is a forced outward movement of the heat panels, which can result in a barrelling of the container.
Thus, Hayashi et al, U.S. Pat. No. 4,877,141, discloses a panel configuration that accommodates an initial, and natural, outward flexing caused by internal hydraulic pressure and temperature, followed by inward flexing caused by the vacuum formation during cooling. Importantly, the panel is kept relatively flat in profile, but with a central portion displaced slightly to add strength to the panel but without preventing its radial movement in and out. With the panel being generally flat, however, the amount of movement is limited in both directions. By necessity, panel ribs are not included for extra resilience, as this would prohibit outward and inward return movement of the panel as a whole.
Krishnakumar et al, U.S. Pat. No. 5,908,128 discloses another flexible panel that is intended to be reactive to hydraulic pressure and temperature forces that occur after filling. Relatively standard ‘hot-fill’ style container geometry is disclosed for a ‘pasteurizable’ container. It is claimed that the pasteurization process does not require the container to be heat-set prior to filling, because the liquid is introduced cold and is heated after capping. Concave panels are used to compensate for the pressure differentials. To provide for flexibility in both radial outward movement followed by radial inward movement however, the panels are kept to a shallow inward-bow to accommodate a response to the changing internal pressure and temperatures of the pasteurization process. The increase in temperature after capping, which is sustained for some time, softens the plastic material and therefore allows the inwardly curved panels to flex more easily under the induced force. It is disclosed that too much curvature would prevent this, however. Permanent deformation of the panels when forced into an opposite bow is avoided by the shallow setting of the bow, and also by the softening of the material under heat. The amount of force transmitted to the walls of the container is therefore once again determined by the amount of flex available in the panels, just as it is in a standard hot-fill bottle. The amount of flex is limited, however, due to the need to keep a shallow curvature on the radial profile of the panels. Accordingly, the bottle is strengthened in many standard ways.
Krishnakumar et al, U.S. Pat. No. 5,303,834 discloses still further ‘flexible’ panels that can be moved from a convex position to a concave position, in providing for a ‘squeezable’ container. Vacuum pressure alone cannot invert the panels, but they can be manually forced into inversion. The panels automatically ‘bounce’ back to their original shape upon release of squeeze pressure, as a significant amount of force is required to keep them in an inverted position, and this must be maintained manually. Permanent deformation of the panel, caused by the initial convex presentation, is avoided through the use of multiple longitudinal flex points.
Krishnakumar et al, U.S. Pat. No. 5,971,184 discloses still further ‘flexible’ panels that claim to be movable from a convex first position to a concave second position in providing for a grip-bottle comprising two large, flattened sides. Each panel incorporates an indented ‘invertible’ central portion. Containers such as this, whereby there are two large and flat opposing sides, differ in vacuum pressure stability from hot-fill containers that are intended to maintain a generally cylindrical shape under vacuum draw. The enlarged panel side walls are subject to increased suction and are drawn into concavity more so than if each panel were smaller in size, as occurs in a ‘standard’ configuration comprising six panels on a substantially cylindrical container. Thus, such a container structure increases the amount of force supplied to each of the two panels, thereby increasing the amount of flex force available.
Even so, the convex portion of the panels must still be kept relatively flat, however, or the vacuum force cannot draw the panels into the required concavity. The need to keep a shallow bow to allow flex to occur was previously described by Krishnakumar et al in both U.S. Pat. No. 5,303,834 and U.S. Pat. No. 5,908,128. This in turn limits the amount of vacuum force that is vented before strain is placed on the container walls. Further, it is generally considered impossible for a shape that is convex in both the longitudinal and horizontal planes to successfully invert, anyhow, unless it is of very shallow convexity. Still further, the panels cannot then return back to their original convex position again upon release of vacuum pressure when the cap is removed if there is any meaningful amount of convexity in the panels. At best, a panel will be subject to being ‘force-flipped’ and will lock into a new inverted position. The panel is then unable to reverse in direction as there is no longer the influence of heat from the liquid to soften the material and there is insufficient force available from the ambient pressure. Additionally, there is no longer assistance from the memory force that was available in the plastic prior to being flipped into a concave position. Krishnakumar et al U.S. Pat. No. 5,908,128 previously disclose the provision of longitudinal ribs to prevent such permanent deformation occurring when the panel arcs are flexed from a convex position to one of concavity. This same observation regarding permanent deformation was also disclosed in Krishnakumar et al U.S. Pat. No. 5,303,834. Hayashi et al U.S. Pat. No. 4,877,141 also disclosed the necessity of keeping panels relatively flat if they were to be flexed against their natural curve.
The principal mode of failure in prior art containers is believed by the applicant to be non-recoverable buckling of the structural geometry of the container, due to weakness, when there is a vacuum pressure inside the container, and especially when such a container has been subjected to a lowering of the material weight for commercial advantage.
The present invention in contrast, allows for increased flexing of the vacuum panel side walls so that the pressure on the containers may be more readily accommodated. Reinforcing ribs of various types and location may still be used, as described above, to still compensate for any excess stress that must inevitably be present from the flexing of the container walls into the new ‘pressure-adjusted’ condition by ambient forces.